1. Field of the Invention
The present invention relates to coating compositions which form a transparent cross-linked hardened film high in abrasion resistance even under small loads, surface smoothness, flexibility, heat resistance, solvent resistance, durability and adhesion to the coated base. These coating compositions may be cured by irradiation with ultraviolet rays in air. This invention also relates to a process for producing synthetic resin moldings improved in the abrasion resistance and surface smoothness using these coating compositions.
2. Description of the Prior Art
Moldings produced from resins such as polymethyl methacrylate, polycarbonate and polyallyldiglycol carbonate are not only lighter in weight and higher in shock resistance, but are also less expensive and easier to mold than glass products and are therefore used in many fields such as organic glass plates, illuminating device covers, optical lenses, spectacle lenses, reflectors, mirrors, sign boards, displays, artificial tooth, name plates, dust cover cases and automobile parts where glass would otherwise have been used.
However, conventional synthetic resin moldings have a serious defect in that they are so deficient in surface abrasion resistance that their surfaces will be damaged by contact with other objects, shocks and scratches during the transportation of the molded articles.
In order to improve the abrasion resistance, it has been suggested to coat the surface of these synthetic resin products with a silicone or melamine-type coating material which is then hardened. However, such techniques suffer from several defects. For example, in both techniques, a thermosetting method is used and therefore in order to harden the coating material, it is necessary to heat it at a high temperature for a long time. Therefore, the apparatus must be large, the productivity will be reduced and the production cost will be high. As to performance, in the former technique, the adhesion of the coating material to the base is not adequate and cracks are likely to occur at the time of working or using the product. In the latter technique, the abrasion resistance of the film is not sufficient and its adhesion to the base is low.
In addition to the techniques mentioned above, there is a procedure wherein a polyfunctional acrylate or methacrylate monomer having at least three polymerizable (meth)acryloyloxy groups in one molecule is applied as a coating material to the surfaces of the synthetic resin molding and which is irradiated with active energy rays to form a cross-linked hardened film by free radical polymerization on the surface of the synthetic resin molding.
The polymerization activity of these polyfunctional (meth)acrylate (meaning acrylate or methacrylate hereinafter) monomer is so high where irradiated with active energy rays that it has been suggested as a quick drying ink material in U.S. Pat. Nos. 3,661,614; 3,551,311; and 3,551,246 and British Pat. No. 1,198,259. Use of such polyfunctional (meth)acrylate monomer as a surface modifier of synthetic resin moldings has been suggested in U.S. Pat. Nos. 3,552,986; 2,413,973; and 3,770,490.
A method of improving the abrasion resistance of synthetic resin molded surfaces by using a polyfunctional (meth)acrylate having at least three (meth)acryloyloxy groups has been suggested in U.S. Pat. Nos. 3,968,304 or 3,968,309. In these procedures, a film is formed without using an organic solvent and it is hardened by irradiation with active energy rays in an inert gas atmosphere.
It has been discovered that polyfunctional (meth)acrylate monomers are effective in improving the abrasion resistance of the surface of a synthetic resin molding as disclosed, for example, in U.S. patent application Ser. No. 878,830, filed Feb. 17, 1978, and now U.S. Pat. No. 4,199,421.
However, the polyfunctional monomers used in such techniques have high viscosities which make it difficult to obtain a smooth coating on synthetic resin molded products. This is a particular problem where the final product is to be used in optical applications, where its value is reduced because of the uneven surface.
Accordingly, a need exists for a composition which can be used to coat synthetic resin articles to improve their surface hardness. In particular, for a composition which forms smooth surface coatings. An additional need exists for a coating composition which does not whiten or become opaque when applied to molded resin articles.